Transparent polyamide-imide resin and film using same

ABSTRACT

Disclosed are a transparent polyamide-imide resin and a film using the same, which can be colorless and transparent, can show excellent thermal stability and mechanical properties, and can have low birefringence, making it possible to serve in various fields including a semiconductor insulator, a TFT-LCD insulator, a passivation layer, a liquid crystal alignment layer, materials for optical communication, a protective film for a solar cell, a flexible display substrate and the like.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a reissue application of U.S. Pat. No. 9,580,555,which was filed as U.S. patent application Ser. No. 15/107,634 onDecember 26, 2014 and issued on Feb. 28, 2017, which issued fromNational Stage State of International Application No. PCT/KR2014/012882,filed Dec. 26, 2014, claiming priority based on Korean PatentApplication Nos. 10-2013-0164009 filed on Dec. 26, 2013 and10-2014-018826 filed on Dec. 24, 2014, the contents of all of which areincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a transparent polyamide-imide resin anda film using the same and, more particularly, to a transparentpolyamide-imide resin and a film using the same, which may exhibitsuperior thermal stability and mechanical properties and lowbirefringence, making it possible to serve as a substrate for a plasticdisplay.

BACKGROUND ART

Generally, a polyimide film is formed from a polyimide resin. Such apolyimide resin is a highly heat-resistant resin prepared by subjectingan aromatic dianhydride and an aromatic diamine or an aromaticdiisocyanate to solution polymerization, thus preparing a polyamic acidderivative, which is then subjected to a ring-closing reaction anddehydration at a high temperature so as to be imidized.

In the preparation of the polyimide resin, examples of the aromaticdianhydride may include pyromellitic dianhydride (PMDA),biphenyltetracarboxylic dianhydride (BPDA), etc., and examples of thearomatic diamine may include oxydianiline (ODA), p-phenylenediamine(p-PDA), m-phenylenediamine (m-PDA), methylenedianiline (MDA),bisaminophenylhexafluoropropane (HFDA), etc.

Since a polyimide resin is a very strongly heat-resistant resin, whichis insoluble and infusible, and is superior in terms of thermaloxidation resistance, heat resistance, radiation resistance,low-temperature characteristics, chemical resistance and the like, ithas been utilized in a variety of fields including those of advancedheat-resistant materials, such as automotive materials, aircraftmaterials, spacecraft materials, etc., and electronic materials such asinsulation coating materials, insulating films, semiconductors,electrode protective films for TFT-LCDs, etc.

However, a polyimide resin is brown- or yellow-colored, attributable toits high aromatic ring density, and thus has low transmittance in thevisible light range, and also, is unsuitable for use in an opticalmember due to the high birefringence thereof.

In order to impart transparency to a polyimide having a deep brown andyellow color, a linkage group (—O—, —SO₂—, —CO—, —CF₃CCF₃—) or a sidechain having a relatively large free volume is introduced to the mainchain, thus minimizing the formation of an intermolecular orintramolecular charge transfer complex, whereby transparency may berealized.

However, such a transparent polyimide film may have decreased heatresistance due to the introduced functional group. This is considered tobe due to the charge transfer complex, and the film becomes transparentbut its heat resistance is decreased. When heat resistance is decreasedin this way, the transparent polyimide film is difficult to apply toadvanced material fields including displays or semiconductors, whichrequire high processing temperatures. To solve this problem, attemptshave been made to polymerize monomers in the solvent after purification,but without any significant increase in transmittance.

U.S. Pat. No. 5,053,480 discloses the use of an alicyclic dianhydridecomponent instead of an aromatic dianhydride. The formation of asolution or a film is improved in transparency and color compared to thepurification method, but the increase in transmittance is limited andthus unsatisfactory transmittance results. Also, the thermal andmechanical properties are deteriorated.

U.S. Pat. Nos. 4,595,548, 4,603,061, 4,645,824, 4,895,972, 5,218,083,5,093,453, 5,218,077, 5,367,046, 5,338,826, 5,986,036, and 6,232,428,and Korean Patent Application Publication No. 2003-0009437 disclose thepreparation of a novel polyimide having improved transmittance and colortransparency in the range within which thermal properties are notsignificantly deteriorated using a linkage group such as —O—, —SO₂—,CH₂—, etc., a monomer having a bent structure connected to an m-positionrather than a p-position, or aromatic dianhydride and aromatic diaminemonomers having a substituent such as —CF₃, etc. However, the abovepolyimide has high birefringence.

Meanwhile, a conventional glass substrate is difficult to realizeflexible properties and may be easily broken, making it difficult to usein real-world applications. To manufacture a thin lightweight substrate,a conventional glass substrate is coated with a polyimide material,after which the glass is separated, or it is formed on a polyimide film,in addition to the use of the thin glass substrate. When a colorlesstransparent polyimide film is applied to display fields, it may beutilized for display devices having various shapes, may exhibit flexibleproperties, and is thin, lightweight and unbreakable.

Therefore, a transparent polyimide, which is to be applied to displayprocesses, needs superior thermal stability that may endure displayprocessing, high mechanical properties for preventing the breakagethereof, and low birefringence to ensure a desired viewing angle.

DISCLOSURE Technical Problem

Accordingly, the present invention is intended to provide a transparentpolyamide-imide resin and a film using the same, which are suitable foruse in a substrate for a plastic display owing to superior thermalstability and mechanical properties and low birefringence thereof.

In addition, the present invention is intended to provide a substratefor a plastic display, which has increased thermal stability andmechanical properties and low birefringence.

Technical Solution

An embodiment of the present invention provides a polyamide-imide resin,which is an imide of a polyamic acid resulting from copolymerizing anaromatic dianhydride and an aromatic dicarbonyl compound with anaromatic diamine, wherein the aromatic dicarbonyl compound is containedin an amount of 1 to 50 mol % based on the total molar amount of thearomatic dianhydride and the aromatic dicarbonyl compound, the aromaticdianhydride includes (i) 4,4′-hexafluoroisopropylidene diphthalicanhydride (6FDA) and (ii) at least one selected from amongcyclobutanetetracarboxylic dianhydride (CBDA) andcyclopentanetetracarboxylic dianhydride (CPDA), and the aromatic diamineincludes 2,2′-bis(trifluoromethyl)-1,1′-biphenyl-4,4′-diamine (TFDB).

In a preferred embodiment of the present invention, the aromaticdicarbonyl compound may include at least one selected from the groupconsisting of p-terephthaloyl chloride (TPC), terephthalic acid,iso-phthaloyl dichloride, and 4,4′-benzoyl chloride4,4′-biphenyldicarbonyl chloride.

In a preferred embodiment of the present invention, the (ii) at leastone selected from among the cyclobutanetetracarboxylic dianhydride(CBDA) and the cyclopentanetetracarboxylic dianhydride (CPDA) may becontained in an amount of 10 to 30 mol % based on the total molar amountof the aromatic dianhydride and the aromatic dicarbonyl compound.

In a preferred embodiment of the present invention, the aromatic diaminemay further include at least one selected from the group consisting ofoxydianiline (ODA), p-phenylenediamine (pPDA), m-phenylenediamine(mPDA), bis (aminohydroxyphenyl)hexafluoropropane (DBOH), bis(aminophenoxy)benzene (133APB, 134APB, 144APB), bis(aminophenyl)hexafluoropropane (33-6F, 44-6F), bis (aminophenyl)sulfone(4DDS, 3DDS), bis[(aminophenoxy) phenyl]hexafluoropropane (4BDAF),bis[(aminophenoxy) phenyl]propane (6HMDA), and bis(aminophenoxy)diphenylsulfone (DBSDA).

In a preferred embodiment of the present invention, the aromaticdianhydride may further include at least one selected from the groupconsisting of biphenyltetracarboxylic dianhydride (BPDA),bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BTA),4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylicdianhydride (TDA), pyromellitic dianhydride,1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), benzophenonetetracarboxylic dianhydride (BTDA), bis(carboxyphenyl) dimethylsilanedianhydride (SiDA), oxydiphthalic dianhydride (ODPA),bis(dicarboxyphenoxy) diphenylsulfide dianhydride (BDSDA), sulfonyldiphthalic anhydride (SO2DPA), and (isopropylidenediphenoxy)bis(phthalicanhydride) (6HDBA).

In a preferred embodiment of the present invention provides apolyamide-imide resin, which is an imide of a polyamic acid resultingfrom copolymerizing an dianhydride and an aromatic dicarbonyl compoundwith an aromatic diamine, the dianhydride includes (i)4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and (ii) atleast one selected from among cyclobutanetetracarboxylic dianhydride(CBDA) and cyclopentanetetracarboxylic dianhydride (CPDA), and thearomatic diamine includes2,2′-bis(trifluoromethyl)-1,1′-biphenyl-4,4′-diamine (TFDB).

Another embodiment of the present invention provides a polyamide-imidefilm, manufactured from the above polyamide-imide resin.

In a preferred embodiment of the present invention, the polyamide-imidefilm may have a transmittance of 88% or more, measured at 550 nm, for afilm having a thickness of 8 to 12 μm, and a coefficient of thermalexpansion (CTE) of 13 ppm/° C. or less, measured at 50 to 300° C. usinga thermomechanical analysis method (TMA method).

In a preferred embodiment of the present invention, the polyamide-imidefilm may have a tensile strength of 130 MPa or more for a film having athickness of 8 to 12 μm when measured according to ASTM D882.

In a preferred embodiment of the present invention, the polyamide-imidefilm may have a birefringence of 0.1 or less, an in-plane retardation(Ro) of 1 nm or less, and a thickness-direction retardation (Rth) of 300nm or less at a thickness of 10 μm.

Still another embodiment of the present invention provides a substratefor a plastic display including the above polyamide-imide film.

Advantageous Effects

According to the present invention, a polyamide-imide resin and a filmusing the same are colorless and transparent, show excellent thermalstability and mechanical properties, and have low birefringence, makingthem suitable for use in various fields including a semiconductorinsulator, a TFT-LCD insulator, a passivation layer, a liquid crystalalignment layer, materials for optical communication, a protective filmfor a solar cell,, a flexible display substrate and the like.

BEST MODE

Unless otherwise defined, all the technical and scientific terms usedherein have the same meanings as those typically understood by thoseskilled in the art to which the present invention belongs. Generally,the nomenclature used herein is well known in the art and is typical.

As used herein, when any part “includes” any element, this means thatanother element is not excluded but may be further included unlessotherwise specifically mentioned.

In the foregoing and following description, the term “imidization” isdefined to include “amidization”, and the term “imide” is defined toinclude “amide”.

An aspect of the present invention pertains to a polyamide-imide resin,which is an imide of a polyamic acid resulting from copolymerizing anaromatic dianhydride and an aromatic dicarbonyl compound with anaromatic diamine. In the preparation of the polyamic acid, the aromaticdianhydride includes (i) 4,4′-hexafluoroisopropylidene diphthalicanhydride (6FDA) and (ii) at least one selected from amongcyclobutanetetracarboxylic dianhydride (CBDA) andcyclopentanetetracarboxylic dianhydride (CPDA), and the aromatic diamineincludes 2,2′-bis(trifluoromethyl)-1,1′-biphenyl-4,4′-diamine (TFDB).

Another aspect of the present invention pertains to a polyamide-imidefilm made of the polyamide-imide resin and a substrate for a plasticdisplay including the polyamideimide film.

Hereinafter, a detailed description will be given of the presentinvention.

The present invention addresses a polyamide-imide resin and a film usingthe same, which are suitable for use in a substrate for a plasticdisplay owing to superior thermal stability and mechanical propertiesand low birefringence thereof. The polyamide-imide resin is an imide ofa polyamic acid resulting from copolymerizing an aromatic dianhydride,an aromatic dicarbonyl compound and an aromatic diamine, and a filmusing the same may be provided. As such, the aromatic dianhydrideincludes (i) 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA)and (ii) at least one selected from among cyclobutanetetracarboxylicdianhydride (CBDA) and cyclopentanetetracarboxylic dianhydride (CPDA),and the aromatic diamine includes2,2′-bis(trifluoromethyl)-1,1′-biphenyl-4,4′-diamine (TFDB).

The aromatic dicarbonyl compound may include at least one selected fromthe group consisting of p-terephthaloyl chloride (TPC), terephthalicacid, iso-phthaloyl dichloride, and 4,4′-benzoyl chloride4,4′-biphenyldicarbonyl chloride.

The aromatic dicarbonyl compound has a benzene ring and may thus exhibithigh thermal stability and mechanical properties, but has highbirefringence due thereto. Also, the cycloaliphatic dianhydride, such ascyclobutanetetracarboxylic dianhydride (CBDA),cyclopentanetetracarboxylic dianhydride (CPDA) and the like, has lowbirefringence but may deteriorate thermal stability and mechanicalproperties.

However, when 2,2′-bis(trifluoromethyl)-1,1′-biphenyl-4,4′-diamine(TFDB) is used as the diamine, thermal stability and optical propertiesmay increase. Also, when the aromatic dianhydride, the aromaticdicarbonyl compound and the aromatic diamine are copolymerized inamounts controlled within specific ranges, the thermal stability,mechanical properties and optical properties may be improved and thusbalanced.

In the present invention, the aromatic diamine may further include anadditional aromatic diamine, in addition to bistrifluoromethyl benzidine(TFDB), in the interest of thermal stability and birefringence, and theadditional aromatic diamine may include, but is not limited to, at leastone selected from the group consisting of oxydianiline (ODA),p-phenylenediamine (pPDA), m-phenylenediamine (mPDA),bis(aminohydroxyphenyl)hexafluoropropane (DBOH),bis(aminophenoxy)benzene (133APB, 134APB, 144APB),bis(aminophenyl)hexafluoropropane (33-6F, 44-6F),bis(aminophenyl)sulfone (ODDS, 3DDS), bis[(aminophenoxy)phenyl]hexafluoropropane (4BDAF), bis[(aminophenoxy)phenyl]propane (6HMDA), and bis(aminophenoxy)diphenylsulfone (DBSDA).

In the present invention, the aromatic dianhydride may further includean additional aromatic dianhydride, in addition to4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and at leastone selected from among cyclobutanetetracarboxylic dianhydride (CBDA)and cyclopentanetetracarboxylic dianhydride (CPDA), in the interest ofthermal stability, mechanical properties and optical properties, and theadditional aromatic dianhydride may include, but is not limited to, atleast one selected from the group consisting of biphenyltetracarboxylicdianhydride (BPDA), bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylicdianhydride (BTA),4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylicdianhydride (TDA), pyromellitic dianhydride,1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), benzophenonetetracarboxylic dianhydride (BTDA), bis(carboxyphenyl) dimethylsilanedianhydride (SiDA), oxydiphthalic dianhydride (ODPA),bis(dicarboxyphenoxy) diphenyl sulfide dianhydride (BDSDA), sulfonyldiphthalic anhydride (SO2DPA), and (isopropylidenediphenoxy)bis(phthalicanhydride) (6HDBA).

The polyamide-imide resin according to the present invention is obtainedby polymerizing an aromatic diamine with an aromatic dianhydride and anaromatic dicarbonyl compound, followed by imidization. In order toattain desired thermal stability, mechanical properties andbirefringence, the aromatic dianhydride and the aromatic dicarbonylcompound are copolymerized with the diamine at an equivalent ratio of1:1, thus preparing a polyamic acid solution. The polymerizationconditions are not particularly limited, but polymerization ispreferably performed at −10 to 80° C. for 2 to 48 hr in an inertatmosphere.

In the present invention, a solvent may be used for solutionpolymerization of the monomers, and is not particularly limited so longas it dissolves polyamic acid, and preferably includes at least onepolar solvent selected from among m-cresol, N-methyl-2-pyrrolidone(NMP), dimethylformamide (DMF), dimethylacetamide (DMAc),dimethylsulfoxide (DMSO), acetone, and diethyl acetate ethyl acetate. Inaddition thereto, a low-boiling-point solvent, such as tetrahydrofuran(THF) or chloroform, or a solvent characterized by low absorption, suchas γ-butyrolactone, may be utilized.

The amount of the solvent is not particularly limited, but is preferably50 to 95 wt %, and more preferably 70 to 90 wt %, based on the totalamount of the polyamic acid solution, in order to obtain a polyamic acidsolution having appropriate molecular weight and viscosity.

Since the amount of the aromatic dicarbonyl compound that is added forthe reaction may affect the thermal stability, mechanical properties andbirefringence of the resin and film, the aromatic dicarbonyl compound isadded in an amount of 1 to 50 mol %, and preferably 5 to 50 mol %, basedon the total molar amount of the aromatic dianhydride and the aromaticdicarbonyl compound so as not to deteriorate the inherent properties ofthe corresponding polyamide-imide.

If the amount of the aromatic dicarbonyl compound exceeds 50 mol % basedon the total molar amount of the aromatic dianhydride and the aromaticdicarbonyl compound, the thermal stability and mechanical properties mayincrease but the optical properties, such as a yellow index ortransmittance, may decrease. In particular, the birefringence value mayincrease, making it difficult to use the resulting film as a displaysubstrate.

On the other hand, if the amount of the aromatic dicarbonyl compound isless than 1 mol % based on the total molar amount of the aromaticdianhydride and the aromatic dicarbonyl compound, the optical propertiesmay increase but the thermal stability and mechanical properties maydecrease, and thus twisting and breakage may occur in the displayfabrication process.

Of the aromatic dianhydride, (ii) at least one selected from amongcyclobutanetetracarboxylic dianhydride (CBDA) andcyclopentanetetracarboxylic dianhydride (CPDA) is used in an amount of10 to 30 mol % based on the total molar amount of the aromaticdianhydride and the aromatic dicarbonyl compound, thereby uniformlyimproving optical properties in the intended wavelength range anduniformly increasing thermal stability and mechanical properties.

The polyamic acid solution thus obtained is imidized to yield apolyamide-imide resin. The useful imidization method may beappropriately selected from among known imidization processes, examplesof which include thermal imidization, chemical imidization, or acombination of thermal imidization and chemical imidization.

The polyamide-imide film may be obtained by casting the polyamic acid ona support and then performing the above imidization process.

Specifically, chemical imidization is performed by adding the polyamicacid solution with a dehydrating agent including an acid anhydride suchas acetic anhydride, etc., and an imidization catalyst including atertiary amine such as isoquinoline, β-picoline, pyridine, etc. Also,thermal imidization or a combination of thermal imidization and chemicalimidization may be controlled or varied depending on the kind ofpolyamic acid solution, the thickness of the resulting polyamide-imidefilm, etc.

More specifically, the polyamide-imide film is manufactured using acombination of thermal imidization and chemical imidization in a mannerin which the polyamic acid solution is added with a dehydrating agentand an imidization catalyst, cast on a support, heated at 80 to 200° C.,and preferably 100 to 180° C., to activate the dehydrating agent and theimidization catalyst, partially cured and dried, and then heated at 200to 400° C. for 5 to 400 sec, thereby obtaining a polyamide-imide film.

Alternatively, the polyamide-imide film may be manufactured from thepolyamic acid solution as follows. Specifically, the polyamic acidsolution is imidized, after which the imidized solution is added to thesecond solvent, precipitated, filtered and dried, thus obtaining apolyamide-imide resin solid, which is then dissolved in the firstsolvent to prepare a polyamide-imide solution, following by afilm-forming process, resulting in a desired film.

When the polyamic acid solution is imidized, the imidization process,such as thermal imidization, chemical imidization, or a combination ofthermal imidization and chemical imidization as mentioned above, may beperformed. In the imidization process through a combination of thermalimidization and chemical imidization, the obtained polyamic acidsolution is added with a dehydrating agent and an imidization catalystand heated at 20 to 180° C. for 1 to 12 hr and thus imidized.

The first solvent may be the same as the solvent used uponpolymerization of the polyamic acid solution, and the second solvent maybe a solvent having lower polarity than the first solvent in order toattain the polyamide-imide resin solid. Specific examples of the secondsolvent may include at least one selected from among water, alcohols,ethers, and ketones. The amount of the second solvent is notparticularly limited, and is preferably 5 to 20 times the weight of thepolyamic acid solution.

The polyamide-imide resin solid thus obtained is filtered and is thenpreferably dried at 50 to 120° C. for 3 to 24 hr taking intoconsideration the boiling point of the second solvent.

In the film-forming process, the polyamide-imide solution, in which thepolyamide-imide resin solid is dissolved, is cast on the support, andthen heated for 1 min to 8 hr while the temperature thereof is graduallyincreased in the range from 40 to 400° C., yielding a polyamide-imidefilm.

In the present invention, the polyamide-imide film thus obtained is heattreated once more so as to remove thermal hysteresis and residual stressfrom the film, thus ensuring stable thermal properties of the film. Thisadditional heat treatment is carried out at 300 to 500° C. for 1 min to3 hr, and the film after heat treatment has a residual volatile contentof 5% or less, and preferably 3% or less.

According to the present invention, the obtained polyamideimide resinhas a weight average molecular weight of 150,000 to 180,000, a viscosityof 700 to 900 poise, and a glass transition temperature of 300° C. orhigher.

Also, the polyamide-imide film according to the present invention has atransmittance of 88% or more, measured at 550 nm for a film having athickness of 8 to 12 μm, a yellow index of 5 or less, and a coefficientof thermal expansion (CTE) of ppm/° C. or less, measured at 50 to 300°C. using a thermomechanical analysis method (TMA method).

Also, the polyamide-imide film according to the present invention has atensile strength of 130 MPa or more for a film having a thickness of 8to 12 μm upon measurement based on ASTM D882, a birefringence of 0.1 orless, an in-plane retardation (Ro) of 1 nm or less, and athickness-direction retardation (Rth) of 300 nm or less at a thicknessof 10 μm.

As mentioned above, the polyamide-imide film according to the presentinvention is colorless and transparent, shows excellent thermalstability and mechanical properties, and has low birefringence, and canthus be useful in various fields including a semiconductor insulator, aTFT-LCD insulator, a passivation layer, a liquid crystal alignmentlayer, materials for optical communication, a protective film for asolar cell, a flexible display substrate and the like.

MODE FOR INVENTION

A better understanding of the present invention may be obtained throughthe following examples, which are set forth to illustrate, but are notto be construed to limit the scope of the present invention.

Example 1

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 716 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 23.99 g (0.054 mol) of 6FDA and 7.06 g (0.036 mol) ofCBDA were added and then stirred for a predetermined period of time andthus dissolved and allowed to react. The temperature of the solution wasthen maintained at 15° C., after which 18.27 g (0.09 mol) of TPC wasadded and allowed to react at 25° C. for 12 hr, thus obtaining apolyamic acid solution having a solid content of 13 wt % and a viscosityof 860 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 95 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement, and a weightaverage molecular weight of 174,000 through molecular weightmeasurement.

In the foregoing and following description, the average particle size ofthe polyamide-imide solid powder was determined by measuring theparticle size thereof three times using a particle size analyzer (S3500,Microtrac) and then averaging them. The precipitated solid was dried andthe resulting copolymer powder was used as an analytical sample, and theweight average molecular weight was measured by drying the precipitatedsolid to obtain a copolymer powder which was then dissolved at aconcentration of about 1% in N,N-dimethylacetamide (DMAc), filtered viaa 0.45 μm PTFE syringe filter, injected, and then subjected to GPC (GelPermeation Chromatography).

95 g of the polyamide-imide copolymer in solid powder form was dissolvedin 768 g of DMAc, thus obtaining an 11 wt % solution. The solution thusobtained was applied onto a stainless plate, cast to 100 μm, dried usinghot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at 300° C. for 30min, and then slowly cooled, after which the resulting film wasseparated from the stainless plate, thus obtaining a 10 μm thickpolyamide-imide film, which was then subjected to final heat treatmentat 300° C. for 10 min.

Example 2

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 744 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 31.99 g (0.072 mol) of 6FDA and 7.06 g (0.036 mol) ofCBDA were added and then stirred for a predetermined period of time andthus dissolved and allowed to react. The temperature of the solution wasthen maintained at 15° C., after which 14.62 g (0.072 mol) of TPC wasadded and allowed to react at 25° C. for 12 hr, thus obtaining apolyamic acid solution having a solid content of 13 wt % and a viscosityof 830 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 104 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 163,000 through molecular weightmeasurement.

104 g of the polyamide-imide copolymer in solid powder form wasdissolved in 841 g of DMAc, thus obtaining an 11 wt % solution. Thesolution thus obtained was applied onto a stainless plate, cast to 100μm, dried using hot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at300° C. for 30 min, and then slowly cooled, after which the resultingfilm was separated from the stainless plate, thus obtaining a 10 μmthick polyamide-imide film, which was then subjected to final heattreatment at 300° C. for 10 min.

Example 3

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 803 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 47.98 g (0.108 mol) of 6FDA and 7.06 g (0.036 mol) ofCBDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react. The temperature of the solution was thenmaintained at 15° C., after which 7.31 g (0.036 mol) of TPC was addedand allowed to react at 25° C. for 12 hr, thus obtaining a polyamic acidsolution having a solid content of 13 wt % and a viscosity of 815 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 110 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 157,000 through molecular weightmeasurement.

110 g of the polyamide-imide copolymer in solid powder form wasdissolved in 890 g of DMAc, thus obtaining an 11 wt % solution. Thesolution thus obtained was applied onto a stainless plate, cast to 100μm, dried using hot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at300° C. for 30 min, and then slowly cooled, after which the resultingfilm was separated from the stainless plate, thus obtaining an 11 μmthick polyamide-imide film, which was then subjected to final heattreatment at 300° C. for 10 min.

Example 4

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 846 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 59.97 g (0.135 mol) of 6FDA and 7.06 g (0.036 mol) ofCBDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react. The temperature of the solution was thenmaintained at 15° C., after which 1.83 g (0.009 mol) of TPC was addedand allowed to react at 25° C. for 12 hr, thus obtaining a polyamic acidsolution having a solid content of 13 wt % and a viscosity of 840 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 114 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 172,000 through molecular weightmeasurement.

114 g of the polyamide-imide copolymer in solid powder form wasdissolved in 922 g of DMAc, thus obtaining an 11 wt % solution. Thesolution thus obtained was applied onto a stainless plate, cast to 100μm, dried using hot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at300° C. for 30 min, and then slowly cooled, after which the resultingfilm was separated from the stainless plate, thus obtaining an 11 μmthick polyamide-imide film, which was then subjected to final heattreatment at 300° C. for 10 min.

Example 5

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 719 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 23.99 g (0.054 mol) of 6FDA and 7.57 g (0.036 mol) ofCPDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react. The temperature of the solution was thenmaintained at 15° C., after which 18.27 g (0.09 mol) of TPC was addedand allowed to react at 25° C. for 12 hr, thus obtaining a polyamic acidsolution having a solid content of 13 wt % and a viscosity of 790 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, 40 further stirred at 70° C.for 1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 90 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 151,000 through molecular weightmeasurement.

90 g of the polyamide-imide copolymer in solid powder form was dissolvedin 728 g of DMAc, thus obtaining an 11 wt % solution. The solution thusobtained was applied onto a stainless plate, cast to 100 μm, dried usinghot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at 300° C. for 30min, and then slowly cooled, after which the resulting film wasseparated from the stainless plate, thus obtaining an 11 μm thickpolyamide-imide film, which was then subjected to final heat treatmentat 300° C. for 10 min.

The polyamide-imide film thus obtained was measured to determine thecoefficient of linear thermal expansion at 50 to 300° C. using a TMAmethod. As a result, the coefficient of linear thermal expansion thereofwas found to be 10.2 ppm/° C.

Example 6

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 764 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 31.99 g (0.072 mol) of 6FDA and 7.57 g (0.036 mol) ofCPDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react. The temperature of the solution was thenmaintained at 15° C., after which 14.62 g (0.072 mol) of TPC was addedand allowed to react at 25° C. for 12 hr, thus obtaining a polyamic acidsolution having a solid content of 13 wt % and a viscosity of 780 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 102 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 150,000 through molecular weightmeasurement.

102 g of the polyamide-imide copolymer in solid powder form wasdissolved in 825 g of DMAc, thus obtaining an 11 wt % solution. Thesolution thus obtained was applied onto a stainless plate, cast to 100μm, dried using hot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at300° C. for 30 min, and then slowly cooled, after which the resultingfilm was separated from the stainless plate, thus obtaining a 12 μmthick polyamide-imide film, which was then subjected to final heattreatment at 300° C. for 10 min.

Example 7

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 806 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 47.98 g (0.108 mol) of 6FDA and 7.57 g (0.036 mol) ofCPDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react. The temperature of the solution was thenmaintained at 15° C., after which 7.31 g (0.036 mol) of TPC was addedand allowed to react at 25° C. for 12 hr, thus obtaining a polyamic acidsolution having a solid content of 13 wt % and a viscosity of 790 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 109 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 151,000 through molecular weightmeasurement.

109 g of the polyamide-imide copolymer in solid powder form wasdissolved in 882 g of DMAc, thus obtaining an 11 wt % solution. Thesolution thus obtained was applied onto a stainless plate, cast to 100μm, dried using hot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at300° C. for 30 min, and then slowly cooled, after which the resultingfilm was separated from the stainless plate, thus obtaining a 10 μmthick polyamide-imide film, which was then subjected to final heattreatment at 300° C. for 10 min.

Example 8

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 849 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 59.97 g (0.135 mol) of 6FDA and 7.57 g (0.036 mol) ofCPDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react. The temperature of the solution was thenmaintained at 15° C., after which 1.83 g (0.009 mol) of TPC was addedand allowed to react at 25° C. for 12 hr, thus obtaining a polyamic acidsolution having a solid content of 13 wt % and a viscosity of 815 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 112 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 165,000 through molecular weightmeasurement.

112 g of the polyamide-imide copolymer in solid powder form wasdissolved in 906 g of DMAc, thus obtaining an 11 wt % solution. Thesolution thus obtained was applied onto a stainless plate, cast to 100μm, dried using hot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at300° C. for 30 min, and then slowly cooled, after which the resultingfilm was separated from the stainless plate, thus obtaining an 11 μmthick polyamide-imide film, which was then subjected to final heattreatment at 300° C. for 10 min.

Comparative Example 1

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 701 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 19.99 g (0.045 mol) of 6FDA and 7.06 g (0.036 mol) ofCBDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react. The temperature of the solution was thenmaintained at 15° C., after which 20.10 g (0.099 mol) of TPC was addedand allowed to react at 25° C. for 12 hr, thus obtaining a polyamic acidsolution having a solid content of 13 wt % and a viscosity of 870 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 93 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 178,000 through molecular weightmeasurement.

93 g of the polyamide-imide copolymer in solid powder form was dissolvedin 752 g of DMAc, thus obtaining an 11 wt % solution. The solution thusobtained was applied onto a stainless plate, cast to 100 μm, dried usinghot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at 300° C. for 30min, and then slowly cooled, after which the resulting film wasseparated from the stainless plate, thus obtaining an 11 μm thickpolyamide-imide film, which was then subjected to final heat treatmentat 300° C. for 10 min.

Comparative Example 2

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 725 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 19.99 g (0.045 mol) of 6FDA and 10.59 g (0.036 mol) ofBPDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react. The temperature of the solution was thenmaintained at 15° C., after which 20.10 g (0.099 mol) of TPC was addedand allowed to react at 25° C. for 12 hr, thus obtaining a polyamic acidsolution having a solid content of 13 wt % and a viscosity of 855 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 94 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 170,000 through molecular weightmeasurement.

94 g of the polyamide-imide copolymer in solid powder form was dissolvedin 760 g of DMAc, thus obtaining an 11 wt % solution. The solution thusobtained was applied onto a stainless plate, cast to 100 μm, dried usinghot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at 300° C. for 30min, and then slowly cooled, after which the resulting film wasseparated from the stainless plate, thus obtaining a 10 μm thickpolyamide-imide film, which was then subjected to final heat treatmentat 300° C. for 10 min.

Comparative Example 3

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 861 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 63.97 g (0.144 mol) of 6FDA and 7.06 g (0.036 mol) ofCBDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react at 25° C. for 12 hr, thus obtaining apolyamic acid solution having a solid content of 13 wt % and a viscosityof 800 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 118 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 162,000 through molecular weightmeasurement.

118 g of the polyamide-imide copolymer in solid powder form wasdissolved in 954 g of DMAc, thus obtaining an 11 wt % solution. Thesolution thus obtained was applied onto a stainless plate, cast to 100μm, dried using hot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at300° C. for 30 min, and then slowly cooled, after which the resultingfilm was separated from the stainless plate, thus obtaining a 10 82 mthick polyamide-imide film, which was then subjected to final heattreatment at 300° C. for 10 min.

Comparative Example 4

While nitrogen was passed through a 1 L reactor equipped with a stirrer,a nitrogen injector, a dropping funnel, a temperature controller and acondenser, 864 g of N,N-dimethylacetamide (DMAc) was added into thereactor, the temperature of the reactor was set to 25° C., 57.64 g (0.18mol) of TFDB was dissolved, and the resultant solution was maintained at25° C. Further, 63.97 g (0.144 mol) of 6FDA and 7.57 g (0.036 mol) ofCPDA were added and stirred for a predetermined period of time and thusdissolved and allowed to react at 25° C. for 12 hr, thus obtaining apolyamic acid solution having a solid content of 13 wt % and a viscosityof 720 poise.

The polyamic acid solution was added with 34.17 g of pyridine and 44.12g of acetic anhydride, stirred for 30 min, further stirred at 70° C. for1 hr, cooled to room temperature, and precipitated with 20 L ofmethanol, after which the precipitated solid was filtered, ground andthen dried at 100° C. for 6 hr in a vacuum, yielding 116 g of apolyamide-imide copolymer in solid powder form.

The polyamide-imide solid powder was found to have an average particlesize of 70 to 80 μm through particle size measurement and a weightaverage molecular weight of 150,000 through molecular weightmeasurement.

116 g of the polyamide-imide copolymer in solid powder form wasdissolved in 938 g of DMAc, thus obtaining an 11 wt % solution. Thesolution thus obtained was applied onto a stainless plate, cast to 100μm, dried using hot air at 150° C. for 1 hr, at 200° C. for 1 hr, and at300° C. for 30 min, and then slowly cooled, after which the resultingfilm was separated from the stainless plate, thus obtaining an 11 μmthick polyamide-imide film, which was then subjected to final heattreatment at 300° C. for 10 min.

<Evaluation of Properties>

(1) Transmittance

The transmittance of the film of each of Examples and ComparativeExamples was measured at 550 nm using a UV spectrophotometer (CM-3700d,made by Konica Minolta).

(2) Yellow Index (Y.I.)

The yellow index was measured at 550 nm using a UV spectrophotometer(CM-3700d, made by Konica Minolta) according to ASTM E313.

(3) CTE (Coefficient of Thermal Expansion)

The CTE was measured at 50 to 300° C. using TMA (Diamond TMA, made byPerkin Elmer) through a TMA method, and the heating rate was 10° C./minand a load of 100 mN was applied.

(4) Measurement of Thickness

Five random points on the polyamide-imide film were selected, and thethickness thereof was measured using an Anritsu electronic micrometerhaving an error of ±0.5% or less.

(5) Birefringence

Birefringence was measured three times at 630 nm using a prism coupler(Sairon SPA4000), and the average value thereof was determined.

(6) Tensile Strength

Tensile strength was measured using 5967 made by Instron according toASTM-D882. A test sample had a size of 13 mm×100 mm, and the tensilestrength thereof was measured 7 times under conditions of a load cell of1 KN and a tension rate of 50 mm/min, and the average value thereof,rather than the maximum value and the minimum value, was determined.

(7) Retardation

Retardation was measured using a RETS made by OTSUKA ELECTRONICS. A testsample, having a square shape with a width and length of 1 inch, wasmounted to a sample holder and fixed at 550 nm using a monochromator,and Ro (in-plane retardation) was measured at an incident angle of 0°and Rth (thickness-direction retardation) was measured at an incidentangle of 45°.Ro=(nx−ny)*dRth=[(ny−nz)*d+(nx−nz)*d]/2Here, nx is a refractive index in an x direction, ny is a refractiveindex in a y direction, nz is a refractive index in a z direction, and dis the thickness of the polyamide-imide film in units of 10 μm.

TABLE 1 Tensile Thick. Transmit CTE Bire- strength RetardationComposition Molar ratio (μm) (%) Y.I. (ppm/° C.) fringence (MPa) Ro RthEx. 1 TFDB/6FDA + CBDA + TPC 100/30:20:50 10 89.9 3.6 10.9 0.061 1640.24 297 Ex. 2 TFDB/6FDA + CBDA + TPC 100/40:20:40 10 89.8 3.2 11.40.054 158 0.16 231 Ex. 3 TFDB/6FDA + CBDA + TPC 100/60:20:20 11 89.8 3.011.9 0.047 150 0.10 159 (144) Ex. 4 TFDB/6FDA + CBDA + TPC 100/75:20:511 90.1 2.9 12.7 0.021 131 0.09 142 (129) Ex. 5 TFDB/6FDA + CPDA + TPC100/30:20:50 11 88.1 4.5 10.2 0.074 170 0.27 298 (271) Ex. 6 TFDB/6FDA +CPDA + TPC 100/40:20:40 12 88.4 4.2 10.9 0.061 164 0.17 270 (245) Ex. 7TFDB/6FDA + CPDA + TPC 100/60:20:20 10 88.7 3.7 11.4 0.052 154 0.15 221Ex. 8 TFDB/6FDA + CPDA + TPC 100/75:20:5 11 88.9 3.2 12.6 0.040 135 0.11150 (136) C. Ex. 1 TFDB/6FDA + CBDA + TPC 100/25:20:55 11 89.0 5.0  9.00.116 172 0.30 600 (545) C. Ex. 2 TFDB/6FDA + BPDA + TPC 100/25:20:55 1087.4 4.5  8.5 0.101 185 0.42 580 C. Ex. 3 TFDB/6FDA + CBDA 100/80:20 1089.8 4.7 37.3 0.008  84 0.16 123 C. Ex. 4 TFDB/6FDA + CPDA 100/80:20 1189.3 4.5 35.2 0.012  87 0.17 135 (122)

As is apparent from Table 1, the polyamide-imide films of Examples 1 to8 was colorless and transparent and exhibited low birefringence and highmechanical properties and thermal stability, compared to those of thepolyamide-imide films of Comparative Examples 1 to 4.

All simple modifications or variations of the present invention may beeasily performed by those skilled in the art, and may be incorporated inthe scope of the present invention.

The invention claimed is:
 1. A polyamide-imide resin, which is an imideof a polyamic acid resulting from copolymerizing an aromatic adianhydride and an aromatic dicarbonyl compound with an aromaticdiamine, wherein the aromatic dicarbonyl compound is contained in anamount of 1 to 50 mol % based on a total molar amount of the aromaticdianhydride and the aromatic dicarbonyl compound, the aromaticdianhydride includes (i) 4,4′-hexafluoroisopropylidene diphthalicanhydride (6FDA) and (ii) at least one selected from amongcyclobutanetetracarboxylic dianhydride (CBDA) andcyclopentanetetracarboxylic dianhydride (CPDA), and the aromatic diamineincludes 2,2′-bis(trifluoromethyl)-1,1′-biphenyl-4,4′-diamine (TFDB). 2.The polyamide-imide resin of claim 1, wherein the aromatic dicarbonylcompound includes at least one selected from the group consisting ofp-terephthaloyl chloride (TPC), terephthalic acid, iso-phthaloyldichloride, and 4,4′-benzoyl chloride 4,4′-biphenyldicarbonyl chloride.3. The polyamide-imide resin of claim 1, wherein the (ii) at least oneselected from among the cyclobutanetetracarboxylic dianhydride (CBDA)and the cyclopentanetetracarboxylic dianhydride (CPDA) is contained inan amount of 10 to 30 mol % based on the total molar amount of thearomatic dianhydride and the aromatic dicarbonyl compound.
 4. Thepolyamide-imide resin of claim 1, wherein the aromatic diamine furtherincludes at least one selected from the group consisting of oxydianiline(ODA), p-phenylenediamine (pPDA), m-phenylenediamine (mPDA),bis(aminohydroxyphenyl)hexafluoropropane (DBOH),bis(aminophenoxy)benzene (133APB, 134APB, 144APB), bis(aminophenyl)hexafluoropropane (33-6F, 44-6F), bis (aminophenyl)sulfone(4DDS, 3DDS), bis[(aminophenoxy) phenyl]hexafluoropropane (4B DAF),bis[(aminophenoxy) phenyl]propane (6HMDA), and bis(aminophenoxy)diphenylsulfone (DBSDA).
 5. The polyamide-imide resin of claim 1,wherein the aromatic dianhydride further includes at least one selectedfrom the group consisting of biphenyltetracarboxylic dianhydride (BPDA),bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride (BTA),4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylicdianhydride (TDA), pyromellitic dianhydride,1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), benzophenonetetracarboxylic dianhydride (BTDA), bis(carboxyphenyl) dimethylsilanedianhydride (SiDA), oxydiphthalic dianhydride (ODPA),bis(dicarboxyphenoxy) diphenyl sulfide dianhydride (BDSDA), sulfonyldiphthalic anhydride (SO2DPA), and (isopropylidenediphenoxy)bis(phthalicanhydride) (6HDBA).
 6. A polyamide-imide film, manufactured from thepolyamide-imide resin of claim
 1. 7. The polyamide-imide film of claim6, wherein the polyamide-imide film has a transmittance of 88% or more,measured at 550 nm for a film having a thickness of 8 to 12 μm, and acoefficient of thermal expansion (CTE) of 13 ppm/° C. or less, measuredat 50 to 300° C. using a thermomechanical analysis method (TMA method).8. The polyamide-imide film of claim 6, wherein the polyamide-imide filmhas a tensile strength of 130 MPa or more for a film having a thicknessof 8 to 12 μm when measured according to ASTM D882.
 9. Thepolyamide-imide film of claim 6, wherein the polyamide-imide film has abirefringence of 0.1 or less, an in-plane retardation (Ro) of 1 nm orless, and a thickness-direction retardation (Rth) of 300 nm or less at athickness of 10 μm.
 10. A substrate for a plastic display comprising thepolyamide-imide film of claim
 6. 11. A polyamide-imide film comprisingthe polyamideimide resin of claim
 1. 12. A substrate for a plasticdisplay comprising the polyamide-imide film of claim
 11. 13. Apolyamide-imide resin, which is an imide of a polyamic acid resultingfrom copolymerizing a dianhydride and an aromatic dicarbonyl compoundwith an aromatic diamine, the dianhydride includes (i)4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and (ii) atleast one selected from among cyclobutanetetracarboxylic dianhydride(CBDA) and cyclopentanetetracarboxylic dianhydride (CPDA), and thearomatic diamine includes2,2′-bis(trifluoromethyl)-1,1′-biphenyl-4,4′-diamine (TFDB).